Maintaining Moore’s law with new memristor circuits

A group at HP Labs discovers a type of circuit predicted to exist in 1971, and …

In the past, electronic circuit theory has revolved around three fundamental components: the resistor, the capacitor, and the inductor. Now a fourth has been added to that list, the memristor. First postulated in 1971 by Leon Chua at the University of California at Berkeley, a working example was recently created by Dmitri Strukov and colleagues at HP Labs. This advance could help shrink transistors even further.

Chua suspected that a memristor should exist based primarily on symmetry. There are four fundamental circuit variables: electric current, voltage, charge, and magnetic flux. For these variables, we have resistors to relate current to voltage, capacitors to relate voltage to charge, and inductors to relate current to magnetic flux, but we were missing one to relate charge to magnetic flux. That is where the memristor comes in.

The memristor relates magnetic flux to charge, but once you dive into the math, it actually boils down to a variable resistance as a function of the charge passed through it. As the authors state, "The fact that the magnetic field does not play an explicit role in the mechanism of memristance is one possible reason why the phenomenon has been hidden for so long; those interested in memristive devices were searching in the wrong places."

The group at HP Labs discovered the memristor by looking at a known phenomenon. They knew that the resistance of titanium dioxide changed with exposure to oxygen, a fact that has been used to create oxygen sensors. The memristor created in HP labs is based on a film of titanium dioxide, part of which is doped to be missing some oxygen atoms. It is these crystal defects that allow an electric current to pass through titanium dioxide, thus the more holes, the lower the resistance.

These holes can be driven from one side to the other by passing a charge across the film. This decreases the average resistance of the entire film. By passing a charge in the opposite direction, the holes can be pushed back. This process can be repeated, effectively turning the memristor off and on, or making it a one or a zero. Of course, you might expect that measuring this resistance by applying voltage should change the resistance. The authors avoid this by using an alternating current; as the current is moving back and forth, there is no net change in resistance.

Currently the good folk at HP Labs have exploited this to create simple data storage devices. Using memristors, they have been able to store 100 gigabits on a single die in one square centimeter. That is substantially more than the 16 gigabits for a single flash chip, and a comparable storage density to modern hard drives. In the future, HP thinks they can get that up to a terabit or more per centimeter... with the access speed of DRAM. Clearly, this will vie with other technologies such as IBM's racetrack memory. Of course, storage is only one possible role for memristors.

Memristors could also be useful in creating analog processors. When there is a smaller change in charge, the change in resistance is also smaller. The authors suggest that this could lead to the development of transistors akin to neurons, in which increased use leads to increased conductance.

A final beauty of memristors comes from their response to decreasing size. The smaller the device, the more important memristance becomes. Conventional electronic circuits have ever increasing problems with heat and leakage at smaller sizes, but memristance is proportional to the inverse of the square of the film thickness, so smaller films mean a stronger memristance effect. By developing transistors based on memristors, we may be able to continue scaling down microprocessors for a (relatively) long time to come.